Leap seconds are inserted at the end of June or December as an additional second after 23:59:59 UTC (Universal Time Coordinated).

It is also possible to have a negative leapsecond, where one second is removed, in a case where the Earth is rotating faster, but such a negative second has never been used, and is rather unlikely to be used in the future.

The difference between UTC and TAI was defined as 10 seconds from January 1972 and the first leapsecond was added in June 1972.

Leap seconds were introduced in 1971 to reconcile astronomical time, which is based on the rotation on the Earth, and physical time, which can be measured with amazing accuracy using atomic clocks.

Leap seconds have to be inserted on average every 1–2 years during this century.

Abandoning leap seconds in the international standard time would not cause any problems for computer systems that are not involved in controlling astronomical or aerospace systems, as long as national/regional civilian time zones do the same.

Given that UTC with leap seconds originated in 1972, and that atomic time did not exist before 1955, it is not clear that any meaning dare be attributed to the fractional bits of the NTP clock during most of the first half of the present NTP era.

Note that the effective date of the end of leap seconds proposed by the 2004 contribution of the United States is 2007-12-21 which is less than two months after the conference.

Finally, as a result of the invention of leap seconds, systems designers and the general public have not had to recognize that time-of-day (universal time) and time interval (atomic time) are two distinct and incommensurate quantities.

The leapsecond will be added just before 00:00 UTC between 2005 and 2006, so if you don't live in that timezone it will affect you at some other time during the day, for example just before 7 PM (12/31) in New York (Source).

The first leapsecond was added on June 30, 1972, and the last one was added in 1999.

One arc second of orbit of the planets Jupiter or Pluto is a very large distance travelled and a crucial measure to be made when sending expensive space craft to snoop around the surfaces of those planets.

Scientists are delaying the start of 2006 by the first “leapsecond” in seven years, a timing tweak meant to make up for changes in the Earth’s rotation.

Although it is possible to have a negative leapsecond --that is, a second deducted from Coordinated Universal Time --so far all have been add-ons, reflecting the Earth’s general slowing trend due to tidal braking.

The first leapsecond was added on June 30, 1972, according to NIST, an arm of the U.S. Commerce Department.

But adding these ad hoc "leap seconds" -- the last one was tacked on in 1998-- can be a big hassle for computers operating with software programs that never allowed for a 61-second minute, leading to glitches when the extra second passes.

Eliminating leap seconds will make sextants and sundials slowly become inaccurate, but supporters say that's OK now that the satellite-supported GPS can give exact longitude and latitude bearings to anyone with a receiver.

The U.S. effort to abolish leap seconds is also firmly opposed by Britain, which would further lose status as the center of time.

p-second.htm seems to confirm that: "If an NTP daemon detects a leapsecond announcement, it passes the announcement on to its clients, and notifies its own operating system clock of the upcoming leapsecond, if the operating system is aware of leap seconds.".

This decision is widely complained about to this day, but its also somewhat understandable — it'd be somewhat gross to require libc have access to an up-to-date list of leap seconds just to handle localtime() Therefore the kernel's idea of time_t must be set after each leapsecond event.

Leap seconds synchronise clock time with solar time used by astronomers.

Daniel Gambis, of the Earth Rotation Service in Paris, which decides when to add or subtract leap seconds, told the BBC: "For me, it would be a problem if the Sun were to rise at 4pm or at a different time like noon or midnight.

Over the course of one year, the difference accumulates to almost one second, which is compensated by the insertion of a leapsecond into the scale of UTC with a current regularity of a little less than once per year.

Since the first leapsecond in 1972, all leap seconds have been positive and there were 23 leap seconds in the 34 years to January, 2006.

The confusion arises because some mistake leap seconds for a measure of the rate at which the Earth is slowing.

In a 24/7 world, leap seconds that adjust the timekeeping of atomic clocks to the time based on the rising and setting of the sun are viewed by many technocrats as a nuisance.

Yep, it's a leapsecond moment, one of those rare occasions when clocks around the world take a stutter step in order to conform with the Earth's wobbly, gradually slowing spin.

If 2005 is disappearing too fast for you, just hold on for a second, because this year you have an extra second to pause and reflect on the year before the ball drops and the calendar flips New Year's Eve.

Throwing the leapsecond to the sharks, they say, will make it easier to develop and maintain global positioning satellites (GPS) and computer communications.

They say people who operate GPS and other time-critical gadgets have trouble accounting for leap seconds, which are somewhat unpredictable and thus tough to factor into software.

But in 1972, after the invention of the hyper-accurate atomic clock, timekeepers began slipping in a leapsecond every few years, so atomic-clock time would jibe with time derived from Earth's position in space.

However, leap seconds are added whenever necessary to keep it in step with solar time to within 0.9 seconds.

The Royal Astronomical Society accuses those eager to drop leap seconds of trying to solve the problem of precision timing by "exporting problems" to those who use clock time as a measure for solar time.

One second is the duration of 9,192,631,770 cycles of the frequency of radiation from cesium atoms.

The second step involves testing exposed cells for a specific protein in the cell membrane, the presence of which indicates cells are dying.

From 1972 (when the world went to the current system of atomic timekeeping) until 1999, 22 seconds were added to the world’s time in order to keep atomic time synchronized with Earth& time, as measured by the Earth& spin.

Within 40 seconds “flashover” occurs— that’s when flames completely engulf the room, depleting the oxygen and turning the atmosphere into toxic smoke and other searing gases.